1. Field of the Invention
The present invention relates to nanostructural materials with ultrafine-grained (UFG) structure and enhanced mechanical and biomedical characteristics and, more particularly, to titanium and its alloys that may be used for making medical implants applied in surgery, orthopedics, traumatology, and dentistry, as well as to a technology for processing these materials for forming structures that ensure specific mechanical and biomedical properties.
2. Description of Related Art
It has been known that strength, reliability, and durability of an implant depend on chemical composition, and mechanical and biomedical characteristics of the material it is made of. At the same time, microstructure plays a key role in establishment of strength, plasticity, fatigue, corrodibility, and biocompatibility in a specific material. Depending on the processing method, the microstructure is able to have various phase composition, size and shape of grains, disorientation of their boundaries, the density of dislocations and other crystalline lattice defects, etc. (M. A. Shtremel, Strength of Alloys, part 1: Lattice defects, 280 pp, Moscow, Metallurgy, 1982; M. A. Shtremel, Strength of Alloys, part 2: Deformation, Moscow, MISiS, 1997, 527 pp., pp 82-113).
Commercially pure titanium has been widely used in manufacturing implants for dentistry and traumatology due to its high biocompatibility (D. M. Brunette, P. Tengvall, M. Textor, P. Thomsen, “Titanium in medicine”, Springer, 2001, 1019 pp., pp. 562-570, paragraphs 17.1, 17.2).
Also, Russian patent RU 2146535, A61C 8/00, A61L 27/00, of Mar. 20, 2000, describes a method for manufacturing intraosseous dental implant from titanium. As commercially pure titanium does not possess high strength characteristics, a multilayered bioactive coating is used in this case in order to increase the mechanical strength of the implant. The coating comprises five various layers applied in succession with the help of plasma spraying.
Enhanced mechanical strength of an implant can also be achieved by the use of high titanium-based alloys. For instance, patent KR20020074843, A61L 27/06, A61L 27/00, published on Oct. 4, 2002, discloses a method for making a removable bone prosthesis of titanium alloys Ti6Al4V, Ti5Al2.5Sn, Ti3Al13V11Cr, Ti15Mo5Zr3Tl, or Ti6Al12NbTa. However, the values of biocompatibility of high titanium alloys are considerably lower than those of commercially pure titanium. Prolonged staying of implants made of those alloys in a human body can result in accumulation of toxic elements such as vanadium and chromium [D. M. Brunette, et al. Ibid]. That is why, to enhance biocompatibility and optimize the process of osseointegration, bioinert coating of calcium hydroxyapatite (bone-salt) powder is applied onto the implant surface in a vacuum furnace upon heating up to 800 . . . 1000° C.
So in the above mentioned patents commercially pure titanium is used for making implants, which can stay in a human body for long. Its main disadvantage, however, is moderate mechanical strength. In this connection, in order to enhance the strength properties of an implant, usually special biocompatible coating applied on the product surface or high titanium alloys with enhanced hardness, strength, and fatigue endurance are used. Biocompatibility of the implants from titanium alloys is achieved through application of biocompatible coatings. On the whole, employment of expensive titanium alloys as well as processes of applying biocoatings onto the product surface results in the increase of the implant net cost.
It is known that the formation of ultrafine-grained (UFG) structures, which contain mostly high-angle boundaries, allows getting a unique combination of strength, ductility, and fatigue endurance in metals and alloys. [R. Z. Valiev, I. V. Alexandrov. Bulk nanostructural metallic materials.—M.: IKC “Academkniga”, 2007.—398 pp.].
Also known in the art has been commercially pure titanium with the UFG structure produced by combined techniques of severe plastic deformation [G. Kh. Sadikova, V. V. Latysh, I. P. Semenova, R. Z. Valiev “Influence of severe plastic deformation and thermo mechanical treatment on the structure and properties of titanium” Metal science and heat treatment of metals, No 11 (605), 2005, pp. 31-34]. The microstructure in the cross section of the billet is characterized by equiaxed grains and subgrains of the alpha-phase with a hexagonal close-packed (HCP) lattice with the average size of about 200 nm and high dislocation density. The indicated technical solution is taken as the closest analogue.
However, the structure in the longitudinal section of the billet investigated along the length of the rod in several areas has alpha-phase grains elongated along the direction of deformation with the length-to-width ratio (grain shape coefficient) of 6:1. The inner area of the elongated grains is fragmented mostly by low-angle dislocation boundaries. Material with such a structure is characterized by anisotropy of properties in the longitudinal and cross sections of the billet that has an adverse effect on the service life of medical implants.
There has been known a technique for processing rods of commercially pure titanium (RU patent No 22175685, C22F 1/18, published on Jul. 27, 2000), in which formation of a high-strength state is achieved by the microstructure refinement via equal-channel angular pressing (ECAP) with a subsequent thermo mechanical treatment. The thermo mechanical treatment includes interchange of cold deformation with the degree of 30-90% and intermediate and final annealing in the range of temperatures from 250 to 500° C. for 0.2-2 hours. As a result, an ultrafine-grained structure with the grain size of about 0.1 μm is formed in the rod-shaped billet.
The disadvantages of this method are a high degree of anisotropy in the structure and properties of the rod material due to heterogeneity of grain morphology in the longitudinal and cross sections of the billet, and a substantial fraction of low-angle boundaries. Such material possesses enhanced strength, but limited ductility, which does not provide sufficient resistance to fatigue failure.